1. Field of the Invention
The present invention relates to a field emission display (FED), and particularly, to a method for sealing a cap and a method for fabricating the cap in a vacuumed space.
2. Description of the Related Art
Recently, interests and importance of display is increased as multimedia is developed. For example, a display having smaller weight, volume and power consumption is required in an environment where the mobility is an important feature such as portable information devices, and a display having larger screen and angular field is required when the display is used as a media for transmitting information to the masses. Therefore, in order to satisfy the above requires, light and thin flat panel display should be developed. Cathode ray tube (CRT) which is mainly used as the display device presently has superior function, however, volume and weight are increased as the screen is increased, and has some problems such as high voltage and high power consumption.
Therefore, there are flat panel displays such as liquid crystal display (LCD), plasma display panel (PDP), electro-luminescence (EL) and field emission display (FED) for solving above problems.
Especially, the FED is a triode like the conventional CRT, however, the FED uses an acute cathode, not using a hot cathode. That is, a cold cathode, which emits electrons by quantum mechanical tunnel effect after concentrating high electric field on an emitter, is used.
Therefore, the electron emitted from the emitter is accelerated by voltage applied between the anode and the cathode, and crashed onto a phosphor formed on the anode to radiate the phosphor. Therefore, the FED has a relatively simple electrode structure, and can be operated with high speed by using phosphor radiation due to electron beam, and has advantages such as full-color, full-grayscale, high brightness and high videorate.
FIG. 1 is a perspective view showing a conventional FED.
FIG. 2 is a cross-sectional view showing the conventional FED.
As shown in FIGS. 1 and 2, the conventional FED comprises an upper glass substrate 2 and a lower glass substrate 8, a spacer 40 for supporting vacuum space between the upper and lower glass substrates 2 and 8, and a field emission array 32 formed on the lower glass substrate 8.
The field emission array 32 comprises a cathode electrode 10 and a resistance layer 12 formed on the lower glass substrate 8, a gate insulating layer 14 and an emitter 22 for emitting electrons formed on upper part of the resistance layer 12, and a gate electrode 16 formed on the gate insulating layer 14.
The cathode electrode 10 supplies electric current to the emitter 22, and the resistance layer 12 restricts overcurrent applied from the cathode electrode 10 toward the emitter 22 to supply even electric current to the emitter 22.
The gate insulating layer 14 insulates between the cathode electrode 10 and the gate electrode 16. The gate electrode 16 is used as a fetch electrode for drawing electrons. The spacer 40 supports the upper and lower glass substrates 2 and 8 so as to maintain highly vacuumed status between the upper and lower glass substrates 2 and 8.
In order to display an image, cathode voltage of negative polarity (−) is applied to the cathode electrode 10, and anode voltage of positive polarity (+) is applied to the anode electrode 4. Therefore, when sufficient electric voltages are applied to the cathode electrode 10 and the gate electrode 16, a strong electric field is generated, and electrons 30 are emitted from a tip of the emitter 22 due to the generated electric field in quantum mechanical tunneling effect. Then, the emitted electrons 30 pass a hall of the gate electrode, and crashed onto phosphors of red, green and blue colors to excite the phosphors 6. At that time, visible ray of one of the red, green and blue colors is radiated according to the phosphor 6.
FIG. 3 is a cross-sectional view showing a conventional FED on which a focusing electrode is formed.
As shown therein, a focusing electrode 20 is formed on the gate electrode 16 for focusing the electrons 30 emitted from the emitter 22. The focusing electrode focuses the electrons 30 by being applied focusing voltage of negative polarity (−). Also, a focusing insulating layer 18 is formed between the focusing electrode 20 and the gate electrode 16.
As described above, the conventional FED requires highly vacuumed status in the panel greater than 10−6 Torr due to the operational properties. For example, a distance about sub-micron is maintained between the gate electrode 16 and the emitter 22 and high electric field of 107V/cm is applied therebetween. If the highly vacuumed status is not maintained between the upper and lower glass substrates 2 and 8, the insulation between the gate electrode 16 and the emitter 22 may be broken. That is, neutral particles in the panel are crashed into the electron beam and positive ions are generated. The positive ions are sputtered on the tip of the emitter 22 to degrade the device. Also, the electrons 30 crashed with the neutral particles lose their energies, and therefore, the electrons 30 can not sufficiently excite the phosphor 6, and thereby to lower the brightness.
Packaging processes of the conventional FED according to above structure will be described as follows.
FIG. 4 is a flow chart showing processing orders of vacuum packaging the conventional FED using a vacuum pump in atmosphere.
FIG. 5 is an exemplary view showing process of installing a tube and process of applying sealant for the conventional FED.
As shown in FIGS. 4 and 5, in the tube installing process, a frit glass is applied on the lower glass substrate 8 as a first sealant 52, and after that, a tube 50 is installed (ST2). At that time, the tube 50 is installed on a hall 51 of the lower glass substrate 8.
After that, a spacer 40 is formed on the upper glass substrate 2, and the frit glass is dispensed and dried around the spacer as a second sealant 54 (ST4). Herein, the second sealant 54 is installed to be higher than the spacer 40 as much as a predetermined distance (H1; usually 1 mm˜2 mm), because the height of the frit glass is reduced about 30˜40% in preform sintering.
After the second sealant 54 is dispensed on the glass substrate 2, the second sealant 54 is pre-sintered (ST6).
FIG. 6 is an exemplary view showing a process of preform sintering the sealant conventionally.
The preform sintering process has different sintering temperature curves according to frit materials in order to completely burn out a binder of organic material included in the frit glass. Generally, in the above preform sintering process, a standard process is to hold the second sealant 54 for 30 minutes˜1 hour at about 300° C. temperature. After the second sealant 54 is preform sintered, the upper glass substrate 2 and the lower glass substrate 8 are compressed and aligned to adhere the substrates.
After that, the upper and lower glass substrates 2 and 8 are moved to a heating chamber to sinter the first and second sealant 52 and 54 (ST6).
FIG. 7 is an exemplary view showing a sealant sintering process in the conventional art.
As shown therein, the sintering process is performed at the temperature of 400° C.˜450° C. which is higher than that of the preform sintering after moving the panel into the heating chamber 70. At that time, when the sintering process is performed under atmosphere environment, the cathode electrode 10, the gate insulating layer 14, the gate electrode 16, the emitter 22, the focusing insulation layer 18 and the focusing electrode 20, which emit the electrons in the FED, may be damaged by reacting with oxygen or carbon in the atmosphere. Especially, the metal material such as the emitter 22 can be oxidated easily, and therefore, the luminous characteristic is lowered greatly.
In order to prevent the damage as above, inert gas 58 such as nitrogen and/or argon is supplied into the panel using a tube 56 extended from the heating chamber 70, and therefore, devices of the field emitting array are not reacted with the oxygen.
On the other hand, FIG. 8 is an exemplary view showing sealant sintering process according to the conventional art.
As shown therein, a gas inlet port 60 is formed on a lower end portion of the heating chamber 70 and a gas outlet port 62 is formed on an upper end portion of the heating chamber 70 to flow the inert gas such as the nitrogen and/or the argon into the entire heating chamber 70, and therefore, the materials for emitting the electrons can not be reacted with the oxygen in a high temperature process. Herein, the inducing of the inert gas is made by opening the inlet port for 10˜20 minutes in the state that a valve out of the outlet port is closed to make the inside of heating chamber 70 be the nitrogen and/or argon inert gas atmosphere, and after that, the outer valve is opened to flow the gas continuously.
Under above atmosphere, when the temperature of the panel is maintained as 400˜450° C. temperature for 30 minutes˜1 hour, the first and the second sealants are sintered and the panel sealing is completed. The above conventional sintering method is defined as a atmosphere sealing method. At that time, the height of the second sealant 54 is extracted during the sintering process, and therefore, the height of the frit glass is coincided with that of the spacer 40.
FIG. 9 is an exemplary view showing a getter inserted into the conventional tube.
FIG. 10 is an exemplary view showing a cutting process of the conventional tube.
As shown in FIGS. 9 and 10, after the upper glass substrate 9 and the lower glass substrate 8 are attached, a getter 66 is inserted into the panel through the tube 50 and the pumping process is performed (ST8). That is, as the panel, on which the upper glass substrate 2n and the lower glass substrate 8 are attached, is heated in the heating chamber 70, and at the same time, the inside of the panel is pumped by a vacuum pump 72. Therefore, when the inside of the panel reaches to a desired vacuumed degree, middle portion of the tube 50 is heated by a local heating device 68 to cut off the tube 50, and therefore, the panel is separated from the heating chamber 70.
On the other hand, in a pinch-off process for cutting off the tube 50, the tube 50 which is exposed in atmosphere is cut, and therefore, the vacuumed degree of the panel is lowered. In order to increase the vacuumed degree of the panel having lowered vacuumed degree, high temperature is compressed to the getter 66 located in the panel to activate the getter 66 (S12). When the getter 66 is activated, the vacuumed degree of the panel is increased more than a predetermined level, and therefore, the final panel is completed.
However, the pinch-off process of the FED as described above is performed under the atmosphere, the oxygen is induced through the hole 51. Accordingly, when the oxygen is induced into the panel, the metal material such as the emitter is easily oxidated, and thereby, the life span of the FED is reduced and the luminous characteristic is lowered greatly. Also, color purities are different from respective points due to the oxygen in displaying. Also, since the conventional panel sealing method is performed at high temperature, it takes a lot of times to process. Also, in the process of installing the tube, the tube is attached on the lower glass substrate 8 by the first sealant 52. At that time, the electrodes formed on the lower glass substrate 8 may be contaminated by the organic binder of the first sealant 52.